The present invention relates to calibration, more particularly to methods and systems for calibrating optical (e.g., imaging) devices and other electronic devices involving or utilizing radiant energy such as propagated as electromagnetic waves.
“Passive” imaging devices receive naturally occurring electromagnetic signals that are emanated or reflected by objects. Passive imaging devices are distinguishable from “active” imaging devices, which transmit electromagnetic radiation (e.g., active radar devices transmit radio signals) in order to receive reflected electromagnetic signals (e.g., radio signals) that are informative of objects and their locations. Unlike active imaging devices, passive imaging devices do not transmit electromagnetic radiation.
Passive imaging devices generally operate in one of three regions of the electromagnetic spectrum, viz., the visual (optical) region, the infrared (IR) region, or the millimeter wave (MMW) region. In the electromagnetic spectrum, the IR region is characterized by longer wavelengths than is the visual region (visible spectrum), the IR region extending between the visual region and approximately one millimeter in wavelength. The millimeter wave region is characterized by longer wavelength than is the IR region, and is characterized by shorter wavelengths than is the “radar” (including microwaves and radio waves) region.
Real-world objects are considered to behave as “graybody” transmitters. The actual emission of a material is the “blackbody” value (the total radiation intensity according to the Stefan-Boltzmann law) multiplied by the material's “emissivity,” which is particular to that material and normally depends on factors such as electromagnetic wavelength, temperature, and emission angle. Emissivity, a dimensionless quantity, is a measure of the ability of a material to radiate absorbed energy. A blackbody is a theoretical object characterized by both perfect (100%) absorption and perfect (100%) emission of radiation. A blackbody has an emissivity of one, while real-world objects have an emissivity of less than one. Emissivity thus represents the ratio of energy radiated by a material at a given temperature, to energy radiated by a blackbody at the same temperature.
Many current techniques for calibrating passive imaging devices, such as passive millimeter wave (PMMW) devices, involve “absolute” calibration. An absolute calibration approach requires absolute knowledge of a given material's radiometric properties. Absolute calibration can be advantageous in that the power radiance characterizing a test material can be known with exactitude. Nevertheless, absolute calibration necessitates a trusted calibration methodology for the wavelength of interest.
Alternative calibration techniques involve “relative” calibration (synonymously, “comparative” calibration), according to which the passive imaging device's (e.g., PMMW device's) output is measured relative to a given baseline body. Relative calibration often assumes this baseline body to be the “idealized” blackbody. The resultant calibration errors are thus due, at least in part, to the unknown deviation of the baseline body from the idealized blackbody.